Nanostructured LnBaCo2O6-d (Ln = Sm, Gd) with layered structure for Intermediate Temperature SOFC cathodes

We evaluated for the first time the use of nanostructured layered perovskites of formulae LnBaCo2O6-d with Ln = Sm and Gd (SBCO and GBCO, respetively) as SOFC cathodes, finding promising electrochemical properties in the intermediate temperature range (~700{\deg}C). The synthesis of these nanomaterials, not reported before, was achieved by using porous templates to confine the chemical reagents in regions of about 200 nm and 800 nm. The performance of nanostructured SBCO and GBCO cathodes for the oxygen reduction reaction was analyzed in symmetrical cells using Gd2O3-doped CeO2 (GDC) as electrolyte. For this purpose, nanostructured SBCO and GBCO cathodes were deposited on both sides of the electrolyte by a simple thick-film procedure and evaluated by Electrochemical Impedance Spectroscopy technique under different operating conditions. We found that cathodes synthesized using smaller template pores exhibited better performance. Besides, SBCO cathodes displayed lower area-specific resistance than GBCO ones.


Introduction
Extensive research has been devoted in the last few years to develop novel materials and microstructures for solid oxide fuel cells (SOFC) components to operate in the intermediate temperature (IT) range (500ºC -700ºC) [1,2,3,4,5,6,7]. We have recently shown that the use of nanostructures is beneficial both for electrolytes [8,9] and electrodes [10,11,12]. In the case of the cathode, we developed nanostructured tubes and rods that show impressively low polarization resistance [13,14,15,16].
Typical SOFC cathodes are made mixed ionic and electronic conductors (MIEC) [17]. Among them, layered MIEC perovskites of formula LnBaCo 2 O 6-δ (LnBCO, Ln = lanthanide) [18,19,20] have shown to display fast oxide-ion diffusion due to a reduction of the oxygen bonding strenght that leads to the appearance of channels for fast ion motion [21]. The layered ordering of cations, characterized by a sequence of atomic layers of Ln followed by a layer of Ba, is due to the large difference between ionic radii of Ba 2+ (r Ba2+ ~ 1.61 Å), and Ln 3+ ions (r Sm3+ ~ 1.24 Å, r Gd3+ ~ 1.107 Å, for example).
Considering those results, it would be desirable to develop cathodes in order to combine both benefits, that related with the layered ordering and to the nanostructured character. However, so far and to the best of our knowledge, no studies have been performed on the study of nanostructured layered cathodes of LnBCO. The main reason may be due to the inherent difficulty to retain the nanostructure in layered perovskite materials, at the typically high temperatures needed for their synthesis procedure.
In this work we successfully obtained nanostructured LnBCO cathodes, with Ln = Gd and Sm, following a similar procedure developed to obtain nanostructured perovskites of different compositions [22,23,24]. By that method, we were able to retain both the nanostructured character and the layered structure.
The obtained materials were further evaluated as SOFC cathodes that display excellent electrochemical properties in the IT range. Thus, this work paves the way to develop other nanostructured layered perovskite materials in different research areas.

Experimental Procedure
We synthesized LnBCO nanostructures by using the pore wetting technique [22]. The Area-Specific Resistance (ASR) of the cathodes was determined from Electrochemical Impedance Spectroscopy (EIS) measurements performed with a Gamry 750 potentiost-galvanostat at zero bias. EIS measurements were performed in air, in pure oxygen and in a mixture of 95% of N 2 and 5% of O 2 .

Results and discussion
XRD data for the LnBCO powders treated at 1050ºC (the sintering temperature of the cathodes) are shown in Figure 1. We used the following nomenclature: Cathodes made with SmBaCo 2 O 6 and templates of 200 nm and 800 nm were labelled as SBCO2 and SBCO8, respectively. Similarly, cathodes made with GbBaCo 2 O 6-δ were labelled as GBCO2 and GBCO8. We show the data for samples obtained with templates of 200 nm of diameter, difractograms corresponding to cathodes obtained with templates of 800 nm present the same crystal structure (see Supplementary material). Single phase samples with orthorhombic crystal structure (Pmmm space group) were obtained for both compositions and for both template pore size studied in this work: 200 nm and 800 nm. Our XRD patterns are consistent with those presented in references [25] for Ln: Gd and [26] for Ln: Sm, indicating that both compounds present an ordered layered crystal structure. By analyzing the broadening of Bragg peaks, an average crystallite size of 40 nm was estimated using the Scherrer equation in all cases. This is an important result because, to the best of our knowledge, the electrochemical properties of nanostructured layered LnBCO have not been reported in the literature before. Also, cathodes made with templates of 800 nm display two scales of porosity, they are porous both at the meso-and nanoscales. The former is due to the pore former of the ink vehicle and the latter being a result of the confinement performed by the templates.  Gd-based cathodes than in Sm ones, but the latter displays the lowest ASR. The lowest ASR of each composition is obtained for the precursors synthesized in templates of smaller pores.
In figure 4 we show the Arrhenius plot of the ASR for all cases. We can see that almost all cathodes reach ASR values lower than 1 Ω.cm 2 above 650ºC and of the order of 0.5 Ω-cm 2 at 700ºC. In the inset we sketch the values of ASR as a function of each sample to analyze the influence of composition and nanostructure. At high temperatures cathodes SBCO2 and GBCO2 are clearly those with the best performance, showing that the microstructure dominates the cathodic behavior. On the other hand, on increasing temperature, composition seems to be much significant, as cathodes made with SmBaCo 2 O 6 display lower ASR than cathodes of GdBaCo 2 O 6 for fixed microstructure.
We found that the EIS spectra of our cathodes mainly consist two processes. One dominant process at intermediate and large frequencies, corresponding to a Warburg element as it is usual in cathodes with high oxide ion conductivity, in series with a parallel between a resistor (R 1 ) and a constant phase element(Q 1 ) at low frequencies. No additional contributions were needed in order to fit the data. We confirmed that R W is related with oxide ion conduction and that R 1 is related with dissociative adsorption and

Conclusions
In summary, we obtained nanostructured layered perovskites of SmBaCo 2 O 6 and GbBaCo 2 O 6-δ by a very simple procedure and attached to an electrolyte retaining its original nanostructural character. The obtained cathode is highly porous due to the precursors powder. All cathodes display very low ASR in the IT range, moreover taking into account that they were simply smeared with a brush for deposition. Our results show that smaller scale nanostructures (accomplished by using templates of smaller diameter) display an enhanced performance. The main contribution to ASR is bulk diffusion of oxide ions, with a minor low frequency contribution ascribed to the redox process in the electrode surface and gas phase diffusion.
Besides the promising results in the area of IT-SOFCs, the successful synthesis of nanostructured layered perovskites, paves the way for their use in other applications such as magnetoresistive materials or materials for solar cells.